This invention relates to a probe head for a NMR-spectrometer comprising at least one transmitter unit for generating electromagnetic waves of high frequencies and at least one preamplifier unit for amplification of signals emanating from a sample which has been excited by means of the excitation waves, with a cryogenically cooled primary detection circuit, including at least a first antenna and a first waveguide, whereby said first antenna is connected to said preamplifier unit via said first waveguide.
The object of a NMR-experiment is to observe the electromagnetic radiation being produced by energy transitions of electrons and/or atoms in a sample irradiated by a high frequency field B1 of frequency xcfx890 in a time and space homogeneous magnetic field B0. The frequency xcfx890 of the excitation field B1 is preferably situated within or above the hf-range at or near about 300 MHz to 3 GHz. This electromagnetic radiation is being observed against the general background of electromagnetic noise associated with thermal fluctuations in the primary detection circuit and within the sample or in the environment, too.
One particular problem common to most NMR experiments is the faintness of the signals emanating from the sample being sometimes further attenuated by a reaction in progress or by some parts of the sample absorbing from the signal. In this respect it is continuously being attempted to improve the sensitivity of the probe head and the signal-to-noise ratio.
The field strength of the constant homogeneous magnetic field B0 and the frequency xcfx890 of its concomitant rf field B1 have to be increased to their limits to maximize the signal-to-noise ratio. Simultaneously, as good as possiblke up to superconducting materials are being used in the primary detection circuit at as low as possible temperatures to minimize conduction losses and inherent electromagnetic noise. In addition the primary detection circuit has to be shielded well-proportioned from external noises.
A probe head of the above mentioned kind is known from U.S. Pat. No. 5,258,710. It comprises a first and a second resonator coupled with it, whereby said second resonator is coupled to a transmitter unit and said first resonator encloses the sample. A radio frequency signal is coupled for excitation of the sample through said second resonator to said first resonator and irradiates the sample being enclosed by said first resonator. Thereafter said first resonator acts like a receiving coil transmitting the received signals to said second resonator. The probe head is being cooled cryogenically on the one hand for improving the signal-to-noise ratio by reducing the thermal noise and on the other hand the conductivity of the probe head is increased by the utilization of superconducting materials for improving the strength of the signal.
Furthermore it is known from U.S. Pat. No. 5,751,146 a surface coil being open at one side fabricated of highly conductive material for NMR experiments, whose conductors are at least three to five times as thick as the skin depth. Thus, its high-frequency resistance is not influenced by the dimensions of the conductors.
Although already much effort was made to enhance the signal-to-noise ratio significantly, so far no NMR-spectrometers became known which achieved a gratifying signal-to-noise ratio, in particular from samples emitting negligible signal radiation.
It is therefore an object of the present invention to provide a probe head as described at the beginning with an optimized efficiency of its antennas and preamplifier unit and with a significantly improved signal-to-noise ratio.
This object is being achieved on the one hand by that at least said first waveguide operates in the range of the anomalous skin effect, whereby the mean free path of the charge carriers at least in said first waveguide being larger than the electromagnetic skin-depth, and whereby said primary detection circuit is provided with means for temporarily matching it with its own characteristic impedance.
The anomalous skin effect with its characteristic skin depth xcex4eff, ensues if the mean free path of the charge carriers 1 is becoming larger than the electromagnetic skin depth xcex4em of the electromagnetic field, 1 greater than xcex4em. Conduction electrons may achieve in particular at low temperatures a mean free path in the range of millimeters to centimeters. Under these circumstances an essentially dissipation free wave propagation becomes feasible regarding the material of the waveguide. Therefore the signal received by the antenna may reach the preamplifier unit with out significant reduction for further processing.
In order that such a waveguide with an extreme low resistance attenuation and extreme high quality factor operating as resonator may be charged with a wave in as short as possible time it has to be matched temporarily with its own characteristic impedance. The analogous has to be performed if the energy of the excitation wave has to be dissipated after termination of the excitation of the sample for initiation of signal receiving. For this e.g. the antenna may be transformed into its characteristic impedance by adding in parallel an impedance via a pin-diode.
The particular advantage of such waveguides operating in the range of the anomalous skin effect in comparison with superconducting ones is in particular due to the first having no problems with fluxoids/fluxquanta.
The utilization of waveguide materials, permitting wave propagation at the anomalous skin effect, is equally advantageous for said first as for any further feasible antennas.
Preferably at least said first waveguide should operate under condition of the extreme anomalous skin effect for minimization of the dissipation.
In this correlation metallic conductors with an inherent resistivity ratio of ri greater than ≈103, preferably of ri greater than ≈104 have proven particularly qualified. The inherent resistivity ratio ri=xcfx81RT/xcfx81LT is defined as the ratio of the inherent resistivity of the conductor material at room temperature to the one at low temperatures, preferably at temperaturesxe2x89xa620 K, usually at 4,2K.
Furthermore, the inherent resistivity of the materials should depend as little as possible on the surrounding applied magnetic field, therefore
xcex94xcfx81/xcfx81=(xcfx81(ri,T,Bxe2x89xa00)xe2x88x92xcfx81(ri,T,B=0))/xcfx81(ri,T,B=0)
(with xcfx81 the inherent resistivity depending on ri, T the absolute Temperature and B the applied magnetic field) should be as small as possible, in particular xcex94xcfx81/xcfx81 less than ≈xe2x89xa65 should not be considerably exceeded at Txe2x89xa620 K and at an inherent resistivity ratio of rixe2x89xa7103.
Ultra pure aluminum has proven to be a particularly qualified metal. Aluminum is preferred with a purity of  greater than 99.9999% (6N-aluminum) with very low defect concentration.
However, on principle aluminum with a purity of ≈99.9% is still usable as material for conductors. However it is to be regarded that the surface-resistance Rs of the conductors changes by orders of magnitude on transition from the anomalous to the electromagnetic skin effect.
Also it is of particular advantage if at least the internal and the external conductor surfaces of the first waveguide possess the same inherent conductivity as the interior of the conductor.
On principle this is valid for the entire primary detection circuit. To this aim the conductor material has to be relaxed completely by annealing and/or ageing, and the surfaces of the conductors for instance will be electro polished in order to remove completely the surface layer which was cold worked during fabrication of the waveguides. In addition the conductor surfaces can be passivated.
By these means a surface resistance in the range of 10xe2x88x927 xcexa9 or better can be achieved at operating conditions of T less than 4 K and at a magnetic induction of 11,744 T, just as in particular in ultra pure aluminum a resistivity ratio ri in the range of 105.
In another preferred embodiment said preamplifier unit and said first antenna are switchably interconnected. If during the term of the excitement of the sample this interconnection is interrupted it is ensured that the preamplifier unit will not receive any excitement waves that are received by the antenna. Thus, it is provided for that the signals of the excitement wave which are much stronger than the signals emanating from the sample will not influence the preamplifier unit""s processing of the signals to be measured.
Further, another constructive embodiment is possible which provides for the direction of propagation of the excitement wave being orientated orthogonal to the direction of propagation of the signals to be measured. In this case, it is possible that the preamplifier unit will not receive any signals of the excitement waves during the term of excitement even without interrupting the connection between the antenna and the preamplifier unit.
Helium-II has demonstrated itself to be preferred cryogen. Helium-II is regarded liquid helium below the xcex-line. Among others helium-II by contrast with helium-I shows no inclination for boiling because of its superfluid and heat-superconducting properties. Noise caused by boiling can be avoided at all by using helium-II as cryogen, therefore reducing the total noise level. Thus, a critical heat-gradient of 4.3xc2x710xe2x88x924 K/m can cause a critical heat flux of 6xc2x7104 W/m2 in helium-II. these numbers indicate an effective heat conductivity about 3.5xc2x7105 times that of copper at room temperature.
The operating temperature of this probe head preferably is at about 1.85 K, at which helium-II achieves its maximum in heat-flux.
Helium-II can be used also for a dielectric medium in the first waveguide as well as in all other waveguides and/or the antennas, as its properties with a dielectric constant of ∈r≈1,055 and an inherent resistivity of xcfx81el less than 1013 xcexa9m resemble closely those of vacuum. Thus helium-II can be used bilateral as cryogen and as dielectricum. Nevertheless the use of vacuum for a dielectric medium is not excluded.
Also it is advantageous, if the antenna in the NMR spectrometer is positioned below said first waveguide. Thus, at start-up operations of the spectrometer during cooling-down of the primary detection circuit to operating temperatures helium gas can depart freely upwards towards the cryostat/cryogenerator and no helium bubbles remain within the antenna nor anywhere.
The object of the invention is further achieved for a probe head as described in the beginning by a second wave guide that is coupled in particular via at least one aperture to said first wave guide, said second wave guide being connected with said transmitter unit, and means being provided for coupling the excitation wave from said second waveguide to said first wave guide, by which means the propagation of the excitation wave is suppressed in direction to said preamplifier unit.
Consequently, excitation waves are being coupled into said first waveguide in the direction of the antenna, thus an antenna of the particular design has not only the function of receiving signals from the sample but also has to generate the B1-field for the excitation of the sample.
The particular means for coupling the excitation waves from the second waveguide to the first waveguide ensure, that the preamplifier unit is not being perturbed by the excitation wave which is of much higher intensity in comparison with the signal, thus the preamplifier unit becomes as soon as possible ready to receive the signal after termination of the excitation wave.
By this means it becomes feasible, that the preamplifier unit being calibrated only for reception of very weak high frequency signals will not be perturbed by the high power excitation wave and that this perturbation does not continue into the detection time after elapse of the excitation of the sample. Therefore the dead-time for receiving signals without perturbation becomes very short, thereby the preamplifier unit is becoming ready for receiving in as short a time as possible respectively immediately operative.
Different possibilities exist for cutting off the propagation of the excitation wave into the direction of the preamplifier unit.
The means for coupling in the excitation wave may enclose a xcex/2-detour waveguide operating like a directional coupler.
In this embodiment the excitation wave, which is to be coupled from the second waveguide into the first waveguide is being divided, whereby both partial waves enter separated, in particular by an odd multiple of quarter wavelength (2n+1)xc2x7xcex/4 (n=0,1,2 . . . ) into the first waveguide, such that they are in phase in the direction of the antenna while they are out of phase by xcex/2 in the direction of the preamplifier unit. By this means the first partial will be conducted as excitation wave towards the antenna for excitation of the sample with appropriate transformation and impedance matching and the wave reflected from there will be annihilated by destructive interference with the second partial wave in the direction towards the preamplifier unit.
Another possibility exists by means of short circuiting the first waveguide between the area where the second waveguide couples into the first waveguide and the preamplifier unit, in particular by at least one pin-diode.
Most appropriately, the short circuit is caused in an area of the first waveguide where the standing wave produces a voltage-node, as the oscillating currents produce there a maximum and consequently the propagation of the excitation wave in the first waveguide in the direction towards the preamplifier unit can be stopped effectively.
In another preferred embodiment of this realization the coupling of the excitation wave from the second waveguide into the first waveguide is switchable. By this means the transmitter unit can be isolated from the first detection circuit at any time.
An appropriate switch can be provided by the second waveguide being terminated as an open-circuited line or cavity resonator, which can be short-circuited temporarily by a discharge gap positioned at a distance of nxc2x7xcex/2 from an aperture in the second waveguide on the opposite side to the transmitter unit for coupling into the first waveguide.
The discharge gap can be positioned moreover at the open termination of the second waveguide between the conducting surfaces, in such a way that the termination can be short-circuited by a helium plasma or arc discharge directly at the discharge gap.
However the conducting surfaces of the second waveguides can be short-circuited at their termination also, whereby one of the conducting surfaces is interrupted by the discharge gap directly in front of its short-circuited termination. Furthermore the discharge gap is dimensioned such, that the termination end of the second waveguide is open-circuited if the discharge gap is xe2x80x9copenxe2x80x9d.
That switch depends on the reciprocal interaction of the openxe2x80x94respectively the short-circuitedxe2x80x94termination of the second waveguide with an aperture xcex/2 distant from it for coupling the excitation wave into the first waveguide. A wave propagated into the second waveguide will be reflected at its short-circuited termination, thus generating a standing wave. A voltage node results in the area of the aperture with an oscillating maximum (anti-node) of the magnetic field, able to couple into the first waveguide. If the short-circuited termination is transformed into an open-circuited one, then the wave propagated in the second waveguide will now be reflected at the open termination, therefore the standing wave is shifted by a quarter of a wavelength, xcex/4. Thus, a voltage anti-node results in the area of the aperture of the second waveguide, generating a large amount of isolation.
To avoid unnecessary noise, no control currents for switches are permitted during transmission times of signals. It is therefore advantageous, if no particular control currents are needed for the actuation of the switch for the modification of the phase of the standing wave. The preferred switch will be actuated in case of the helium plasma discharge by a laser, in case of the arc discharge by the excitation wave itself.
At a preferred embodiment of this switch at least one electrode of the discharge gap is refrigerated by helium-II, whereby this electrode makes contact with a channel having at least one inlet and one outlet for helium-II, and whereby the outlet is equipped with a semipermeable diaphragm. As the superfluid helium-II moves in the shortest possible time towards a heat source, the heat will be carried off during its dissipation due to the helium-II flow being directed by the diaphragm.
At another preferred embodiment of this switch the second waveguide branches out into two conducting paths, which couple to the first waveguide preferably (2n+1)xc2x7xcex/4 distant from each other, whereby each of the two conducting paths (13xe2x80x3, 14xe2x80x3) end in a termination which can be shortened by (2n+1)xc2x7xcex/4 by a switchable short circuit.
At a particular embodiment of this switching design both of the conducting paths have at their termination short-circuited length of (2n+1)xc2x7xcex/4 in length, whose entrance in particular can be short-circuited with at least one pin-diode (positive intrinsic negative).
Whereas the excitation wave is being coupled into the first waveguide due to a short-circuit by the pin-diodes at the entrance of the cavities generating these voltage-node, this node will be displaced by xcex/4, if the pin-diodes open the cavities, in such a way that the excitation wave is not anymore able to couple into the first waveguide.
Both conductor paths can be arranged such, that one of them produces in reference to the other one a xcex/2-detour and both branches function together as directional coupler.
Both preferably to be used types of switches can be switched within fractions of a nanosecond, thus an extreme short time for decoupling of the transmitter unit from the detection circuit can result. This is being achieved by the second waveguide respectively any other waveguides can be matched temporarily by its characteristic impedance, thus the waveguide obtains a finite attenuation and can be charged fast (surge-charged) with the excitation wave.
At another preferred embodiment at least one of the waveguides is arranged as coaxial transmission line.
Coaxial transmission lines comprising an inner cylindrical conductor and an outer cylindrical conducting sheath offer the advantage of an uniform and in comparison with the thickness of the sheath smaller skin depth compared with other shapes of waveguides. The electromagnetic fields propagate almost entirely within the intervening dielectric medium, thus avoiding the generation of stray fields. The outer surface of the outermost conducting sheath of the coaxial transmission line offers also a good shield against electromagnetic radiation from the environment.
Several coaxial waveguides can form also a multiple coaxial guide with a common axis. Such a guide exhibits besides a cylindrical central conductor two or more coaxial cylindrical conducting sheaths separated each by a dielectric medium, whereby the exterior surface of any sheath serves for an inner conductor for the inside of the next larger sheath. By this means can be assembled double, triple or multiple coaxial waveguides with a common axis each with a length of nxc2x7xcex/4 and each conducting high-frequency currents independent of each other.
By this means it is feasible to feed into the probe head in addition to the proton-frequency, very frequently needed in NMR experiments, other, preferably lower frequencies by a simple and compact assemblage. The introduction of an excitation wave of lower frequency from an outer coaxial guide into an inner coaxial guide occurs preferably at a null point of the impedance, providing simultaneously a voltage-node of the excitation wave in the inner coaxial line. At this voltage-node a high degree of isolation between the lower frequency channel and the higher frequency channel is being achieved.
Further other frequencies can be coupled into such a waveguide system via appropriate apertures and impedance matching.
The excitation of the sample with pulse sequences of different frequencies is necessary for the determination of contained substances and their structure. Thus the high frequency channel, often proton frequency, is used e.g. for cross polarization and dipolar decoupling, and the lower frequency channels are used for observation, dephasing or coherence transfer.
In this association it is also advantageous, if the first and the second waveguide form a multiple coaxial transmission line with a common axis. By this means it is possible to arrange the waveguides most space-economizing. This is particularly advantageous if as dielectric medium helium-II is being used, which circulates by the Fountain-effect through appropriate positioned semipermeable diaphragms and removes heat adequately. Also is being achieved by the compact assemblage of the waveguides, that the dimensions of the probe head can be kept as small as possible, so that the small space in a superconducting high field magnet is being optimal utilized.
In particular it is possible to achieve an attenuation constant of xcex1xe2x89xa610xe2x88x928 Np/m if the coaxial designed waveguides are constructed of some material enabling power transmission at the anomalous skin effect. By this means such coaxial waveguides are almost lossless.
According to another embodiment of this invention a second antenna is provided for improving the signal-to-noise ratio furthermore substantially, whereby the antennas are arranged such that the first antenna detects the sample and the noise-signal within the equatorial near field of the sample while the second antenna detects only the noise signal within the axial near field of the sample.
A NMR-sample is best described by a Hertzian dipole, being irradiated by the excitation wave and radiating itself. This Hertzian dipole is arranged at the centre of the first antenna, which generates the high frequency B1-field in the x-direction, whereby as well the B1-field as also the Hertzian dipole are oriented in the same direction perpendicular to the z-direction, which is oriented along the longitudinal axis of the first antenna.
At the near field of a Hertzian dipole, is the energy/power radiated due to the excitation at its maximum in the equatorial plane whereas no energy/power is radiated into the axial direction. In opposition the thermal Nyquist-noise is radiated isotropically at all directions by the sample. This nearfield radiation pattern yields a possibility for receiving two independent signals simultaneously from one sample, by measuring at the near-field of the sample the energy/power being radiated at the equatorial plane as well as the one radiated at the axial direction.
The first antenna can be constructed at one preferred embodiment as resonator in the shape of a tube slitted lengthwise on both sides with a termination that can be short-circuited. Such an embodiment corresponds with the slitted, saddle-shaped, u.h.f-single-turn-Helmholtz-coils used at the present time in high-resolution NMR-experiments.
The reactive short-circuit can temporarily be transformed into a characteristic impedance by switching in an impedance for surge-charging and -discharging of the primary detection circuit with the excitation wave.
The antenna can be coupled to the first waveguide via a xcex/4-transformation with frequency bandwidth compensation. By this means the impedance of the first waveguide will be matched almost lossless to the impedance of the antenna, thus perturbing reflection will be avoided.
The second antenna can be realized e.g. by a short electric or magnetic dipole-antenna oriented in the near-field of the sample in the direction of the B1-field in particular with a Luneburg lens focussing.
Each of the different approachesxe2x80x94the utilization of the anomalous skin effect, the switchable coupling of the excitation wave in connection with the isolation of the transmitter unit from the preamplifier unit, the utilization of helium-II as cryogen as well as dielectric as well as the utilization of the near-field characteristic of the radiating sample are qualified to improve significantly the signal-to-noise ratio.
Nevertheless it is to be established, that the object determined at the beginning is achieved optimal by the utilization of all the different approaches combined. Thus improvements in the signal-to-noise ratio can be achieved at NMR-experiments at samples radiating extreme weak signals by a factor far in excess of 50 in comparison with today""s at room temperature operating probe heads.